The architecture of the endoplasmic reticulum (ER) consists of an intricate network of sheets and tubules, but how these domains are generated remains largely unknown. We recently identified a class of membrane proteins, the reticulons and DP1/Yop1p, which shape the tubular ER. We have used a a combination of fluorescence recovery after photobleaching (FRAP), sucrose gradient centrifugation, and cross-linking experiments to show that the reticulons and DP1/Yop1p form oligomers in the tubular ER. This oligomerization appears to become more extensive when ATP is depleted from cells. Oligomerization of the reticulons is mediated through their conserved domain that contains the two membrane-embedded segments. Moreover, mutants of yeast reticulon that no longer localize exclusively to the tubular ER have defects in oligomerization. We have also been able to demonstrate that individual members of the reticulon, DP1/Yop1p family are able to generate membrane tubules in vitro. Our findings suggest that the dynamic oligomerization and depolymerization of these proteins is required to shape the tubular ER in living cells. In a second project we are working on the role a dynamin-like GTPase called Sey1p in ER-ER fusion and ER biogenesis.